1. Field of the Invention
The present invention relates to a differential type variable gain amplifier such as a variable gain differential amplifier.
2. Description of the Related Art
FIG. 2 shows the structure of a conventional variable gain differential amplifier. A balanced signal is input to bases of signal amplifying transistors 7 and 7a differentially connected to a constant current source 8. In addition, variable gain amplifiers A and B are connected to collectors of the signal amplifying transistors 7 and 7a, respectively. The variable gain amplifier A amplifies a positive phase signal and the variable gain amplifier B amplifies a negative phase signal. The variable gain amplifier A has gain control transistors 6 and 10, and the variable gain amplifier B has gain control transistors 10a and 6a. A gain control voltage 12 is applied to bases of the gain control transistors 6 and 6a and a gain control voltage 11 is applied to bases of the gain control transistors 10 and 10a. 
Collectors of the gain control transistors 6 and 6a are connected to a power supply terminal 33 via load resistors 4 and 4a, respectively, and collectors of the gain control transistors 10 and 10a are directly connected to a power supply terminal 34. A wiring line inductor 23 is interposed between the power supply terminal 33 and a power source 1, and a wiring line inductor 24 is interposed between the power supply terminal 34 and the power source 1. In addition, the collector of the gain control transistor 6 is connected to an output terminal 5 and the collector of the gain control transistor 6a is connected to an output terminal 5a. 
In the above-mentioned structure, a signal amplified by the signal amplifying transistors 7 and 7a is subjected to gain control by the variable gain amplifiers A and B. Then, the signal is output in balance to the output terminals 5 and 5a (for example, see Japanese Unexamined Patent Application Publication No. 2001-007667 (FIG. 4)).
However, in the above-mentioned structure, a voltage of the power supply terminal 33 is divided into four voltages, which are supplied to the constant current source 8, the signal amplifying transistors 7 and 7a, the gain control transistors 6 and 6a of the variable gain amplifiers A and B, and the load resistors 4 and 4a, respectively. As a result, low voltages are supplied, and the signal amplifying transistors 7 and 7a and the gain control transistors 6 and 6a operate in low voltages, which consequently results in insufficient performances. To solve this problem, it is necessary to increase the voltage supplied. However, increasing the voltage is inappropriate in portable apparatuses using a battery as a power source.